The present invention relates to making carbon fiber reinforced composite material parts that are densified by a matrix made at least in part by a chemical vapor infiltration (CVI) type process.
A particular, but non-exclusive, application of the invention lies in making brake disks out of carbon/carbon (C/C) composite material, in particular for airplane brakes comprising an assembly of alternating stator and rotor disks on a common axis. Nevertheless, the invention is applicable to making other C/C composite material parts and to making parts out of other carbon fiber reinforced composite materials, in particular out of ceramic matrix composite (CMC) material or out of composite materials having a matrix of both carbon and of ceramic.
It is well known to densify porous substrates, such as fiber substrates, by using a CVI type process.
In a conventional CVI process, substrates for densifying are placed in an oven. A reaction gas is admitted into the oven in order to deposit the matrix-constituting material within the pores of the substrates by decomposition of one or more of the components of the gas or by reaction between a plurality of components under determined conditions, in particular of temperature and pressure.
A major difficulty with CVI type processes lies in minimizing the densification gradient within substrates so as to obtain parts having properties that are as uniform as possible throughout their volume.
While the matrix is being deposited, it tends to build up preferentially in the surface portions of the substrates that are the portions first encountered by the reaction gas. This leads to depletion of the gas that manages to diffuse into the cores of the substrates, and also to premature plugging of the pores in the surface portions of the substrates, thereby progressively diminishing the ability of the gas to diffuse into the cores. Consequently, a densification gradient becomes established between the surface portions and the cores of the substrates.
That is why, in particular when making thick parts, it is necessary in practice, once a certain degree of densification has been achieved, to interrupt the process, withdraw the partially-densified substrates, and then machine their surfaces in an operation referred to as “scalping” so as to re-open their surface pores. Densification can then be continued with the reaction gas having easier diffusion access to the cores of the substrates. For example, when making brake disks, it is common practice to perform at least two CVI densification cycles (cycles I1 and I2) with an intervening scalping operation. In practice, a densification gradient is nevertheless observed in the parts as finally obtained.
In order to avoid generating a densification gradient and to avoid possible scalping operations, it is indeed known to implement a CVI densification method that uses a temperature gradient, i.e. by heating the substrates in non-uniform manner. Non-uniform heating by direct coupling between a susceptor and one or more annular substrates for densification is described in documents U.S. Pat. No. 5,846,611 and EP 0 946 461. Matrix deposition in those zones of the substrates that are less easy for the gas to access is enhanced by raising such zones to a temperature that is higher than the temperature of other portions of the substrates. Nevertheless, that technique is restricted to substrates that are of certain shapes and to substrates that are loaded in the oven in certain arrangements.
Document U.S. Pat. No. 5,405,560 proposes facilitating access for the reaction gas to the cores of substrates constituted by annular fiber preforms for C/C composite material brake disks by providing passages in the form of holes that extend through the preforms between their opposite faces. Those holes are provided by inserting needles that push away the fibers of the preforms without damaging them. During CVI densification, the holes provide the gas with short-cut paths for reaching the central portions of the preforms. Parallel document FR 2 616 779 does indeed also mention the possibility of making holes by means of a fluid under pressure that destroys the fibers in part, but it recommends avoiding damaging the fibers.
In contrast, document WO 2006/129040 proposes making holes in a fiber substrate by removing material by breaking fibers, e.g. by machining using a jet of water under high pressure or by mechanical machining, with the purpose being to conserve a substantially unmodified arrangement for the fibers, in particular in the walls of the holes. Densifying such a substrate by means of a CVI type process is found to be practically uniform, with the extent to which densification gradient is reduced being better than with the method of document U.S. Pat. No. 5,405,560.
Tests performed by the Applicant have shown that the effectiveness with which the fiber substrate is perforated for the purpose of reducing the densification gradient is related to the open area of the substrate, i.e. when considering the face of the substrate into which the holes open out, the percentage of the area that is occupied by the holes compared with the total area of said face. More precisely, increasing the open area leads to a reduction in densification gradient.
For given hole density, i.e. for given number of holes per unit area in the face of the substrate where the holes open out, an increase in the open area can be obtained by increasing the cross-section of the holes.
Beyond a certain threshold, increasing the cross-section of the holes presents drawbacks. The resulting composite material becomes less uniform. Furthermore, at least in certain applications, the presence of holes that are not closed during substrate densification alters the properties of the resulting part. Thus, with a brake disk, tests performed by the Applicant have shown that above a certain threshold, the presence of non-plugged holes in the composite material leads to significantly greater wear of the composite material. This can be explained by the action on the edges of the holes while the brake disk is rubbing against another disk. It is possible to envisage plugging the holes after densification. That is mentioned in above-mentioned document U.S. Pat. No. 5,405,560 which recommends forming holes having a diameter in the range 0.5 millimeters (mm) to 5 mm. Plugging the holes requires an additional operation, which increases fabrication costs and does not prevent the material from being substantially non-uniform.
Increasing the open area by increasing hole density, i.e. the number of holes, raises the problem of making a large number of holes of relatively small cross-section. The Applicant has established that in order to obtain a result that is meaningful in terms of gradient reduction, and thus in terms of reducing the duration of densification, while ensuring wear is analogous to that of airplane brake disks fabricated from substrates having no holes, it is necessary to form more than 1000 holes in the surface of a disk having a diameter of 20 inches. This is difficult to envisage using the technique described in documents U.S. Pat. No. 5,405,560 and WO 2006/129040, if it is desired to achieve a favorable balance between making savings during densification and increasing fabrication costs to make the holes.